A wiper controller conventionally uses a raindrop detection device for automatically controlling a wiping system that removes raindrops from a windshield of a vehicle.
The raindrop detection device uses a light emitted from a light emitting device and received by a light reception device for detecting moisture on the windshield. Japanese Patent Document JP-A-2001-349961 and U.S. Pat. No. 6,433,501 disclose the raindrop detection device that detects wet condition of the windshield due to moisture by measuring and detecting a difference of the amount of the light received by the light reception device. That is, the amount of the light received by the light reception device changes when a surface of the windshield is wet.
Characteristics of the light reception device used in the raindrop detection device changes depending on temperature of a surrounding environment. Therefore, it is important to know how to compensate the changed characteristics of the light reception device by taking the temperature into account. The Japanese Patent Document JP-A-2001-349961 discloses the temperature compensation of the characteristics based on a correlation of the characteristics and the temperature prepared and stored in the light reception device. In this manner, the raindrop detection device improves accuracy of detection of the moisture on the windshield.
Conventional raindrop detection devices have restrictions on the size of their casing because they are disposed on the windshield that is in a view of a driver of the vehicle. Therefore, a detection area of the raindrop detection device has to be maximized in terms of space utilization of projected size of the device. The U.S. Pat. No. 6,433,501 describes the arrangement of the light emitting devices and the light reception devices at the corners of a triangle/parallelogram in its disclosure. In this case, the light emitted from the light emitting device is received by the two light reception devices at the same time to effectively utilize a space in the raindrop detection device.
However, the raindrop detection device disclosed in the Japanese Patent Document JP-A-2001-349961 has to have correlation data over a broad range of temperature to compensate the temperature characteristics of the light reception device prior to the production of the device. The correlation data collection procedure by sampling outputs of the light reception device is not only time-consuming but leads to an increased cost of production. In addition, the data storage device for storing a large amount of the correlation data also contributes to the increase of the cost.
Further, the raindrop detection device disclosed in the U.S. Pat. No. 6,433,501 includes a space that can not be used for raindrop detection at the center of a triangle/parallelogram shape defined by the light emitting devices and the light reception devices at the corners of those figures. Therefore, the utilization of the space in the raindrop detection device in the disclosure is unavoidably restricted.